fix(theming)!: remove background.primary color variable

[ze-flo]

Description

Following @lucijanblagonic's Figma comment, background.primary isn't relevant to Garden consumers and should be removed.

Details

• Removes background.primary from Theme
• Replaces instance of background.primary with background.primaryEmphasis

Checklist

• 👌 design updates will be Garden Designer approved (add the designer as a reviewer)
• 🌐 demo is up-to-date (npm start)
• ⬅️ renders as expected with reversed (RTL) direction
• ⚫ renders as expected in dark mode
• ~~:metal: renders as expected with Bedrock CSS (?bedrock)~
• 💂‍♂️ includes new unit tests. Maintain existing coverage (always >= 96%)
• ♿ tested for WCAG 2.1 AA accessibility compliance
• 📝 tested in Chrome, Firefox, Safari, and Edge

[coveralls]

coverage: 95.904%. remained the same
when pulling 6036eaa on ze-flo/remove-background-primary
into 8c383a0 on main.

[jzempel]

I'm waiting for the PR to deploy, but I don't think this is what we want to demonstrate with this pattern. The idea is to get the avatar's surface to match the active menu item as close as possible. So, it should be something like getColor({ theme, hue: 'primaryHue', light: { shade: 100}, dark: { shade: 900} }) to correspond with the previous variable values.

[jzempel]

Or maybe getColor({ theme, variable: 'background.primaryEmphasis', transparency: 100 }) is a more successful match 🤷

[ze-flo]

Or maybe getColor({ theme, variable: 'background.primaryEmphasis', transparency: 100 }) is a more successful match 🤷

getColor will throw because the output of the ☝️ is rgba() and it's treated as a color variable ($surfaceColor.includes('.')). We would have to refine the logic everywhere this comes up.

ReferenceError: Error: color variable 'rgba(31,115,183,0.08)' is not defined

Also, with the way Avatar is built, the transparent surfaceColor would lay on top of the background color. That wouldn't result in the right color.

[jzempel]

Suggested change

	* `background.primaryEmphasis`) to render based on light/dark mode, or any hex value.
	* `background.emphasis`) to render based on light/dark mode, or any hex value.

[nit] because we don't want to suggest that primaryEmphasis should be used for anything other than primary interactive elements.